Optoma HD20 Projector Screen Recommendations
Since the HD20 is an entry level projector, I'll assume there isn't a huge budget for a screen. Depending on the room, you may choose a fixed wall mount screen (more expensive), a motorized screen (can be more or less than a fixed wall screen, depending on brand), or a low cost pull-down screen.
More important than that is choosing the right surface. There will be two primary questions to ask yourself. How much ambient light will you have to deal with (and where in the room is it coming from), and how large a screen (within reason)? Additionally, what will you be watching - primarily movies, or a mix including lots of TV, both sports and regular programming?
Since from an image standpoint, the black levels of the Optoma HD20 are purely entry level, many will be tempted to go with a high contrast gray projector screen surface. This will lower black levels a bit, a very good thing for this projector when watching movies.
This same type of screen will help with ambient light issues, if the ambient light is coming from the sides, be it your own lighting, or some light leaking in from mostly covered windows.
On the other hand, if you have pretty good lighting control and are, for example, a sports fanatic, and are more concerned with the next NFL game than watching The Dark Knight for the 4th time, then you might want to choose a white surface screen with good gain - 1.3 - 1.5. (An excellent example - and match -would be the fixed wall Carada Brilliant White, which I use in my testing room, for all the images in this review except for some sports shots). You can have even more gain if your seating is close to dead center (the higher the gain, the narrower the good "viewing cone"). Keep in mind, that even if you are sitting dead center, a high gain screen, say 2.3, is going to have the sides, and especially the corners of the image appear noticeably darker than the center.
Again, match your screen to your room, and the type of viewing you are doing. I can't understate how much help a high contrast gray surface can make, though if you are stuck with some side lighting, it's not going to make your system perform like you don't have an ambient light issue. But it really will help, compared to using a white surfaced projection screen.